A variety of technologies are available for acquiring fingerprints, which include mechanical, optical, opto-electrical, electrical, and/or other methods. Mechanical methods emphasize inks and powders to form physical replicas of fingerprints in a flat format. Optical methods utilize traditional film-based photography. Opto-electrical methods include digital photography and a variety of other ways to convert optical images or local optical effects into electronic records. Electrical methods generate electronic fingerprint records without using intermediary optical effects. Such electrical methods generally include a set of sensors that individually monitor changes in local electrical properties in response to interaction with a small region of a finger.
The generation of electronic fingerprint records may be desired to facilitate the rapid communication and analysis of fingerprint information using advanced telecommunications and computer technologies. Both opto-electrical and electrical fingerprinting methods may be used to make fingerprint acquisition systems with flat formats. Flat formats may be desirable to reduce the size, weight, and potentially the cost of the system, and are thereby marketable as portable devices or subcomponents of personal electronics.
Whereas small (i.e. less than about 1 square cm) electrical acquisition arrays may benefit from the economics of large scale silicon based integrated circuit manufacturing techniques, many fingerprinting applications require acquisition arrays that are about 10 cm by 10 cm or larger. Such large silicon based integrated circuits often become prohibitively expensive due to quality control statistics. This may lead to low effective yields of acceptable quality devices.
Many opto-electrical and electrical fingerprinting methods suffer from high variability in the properties of finger tissue and of the finger surface. Finger properties of concern include moisture, salinity, contamination, reflectance, scattering, ambient light, impedance, resistivity/conductivity, and/or other properties. Even within the same fingerprint, these and other properties may vary significantly from person to person, over time, and over distance. Variability in these finger properties may significantly alter the quality and character of a recorded fingerprint.
Explanation of Total Internal Reflection
It is well known that light travels at different speeds in different materials. A refractive index, ni, of a material, i, is the speed of light in a vacuum, c, divided by the velocity of light in the material, vi: ni=c/vi. As light passes from one material to another, the change of speed results in refraction. Measured from perpendicular to the surface, the angle of incidence, θ1, and the angle of refraction, θ2, are given by Snell's law: n1 sin(θ1)=n2 sin(θ2). Accordingly, when light emerges from a glass block (n1˜1.5) into air (n2=1), the light will be refracted towards the surface. That is, θ2>θ1 because n1>n2. At a critical angle of incidence, θc, θ1 becomes 90° as the refracted light runs along the glass-air surface to form an evanescent wave. When θ1>θc, the incident light is reflected back into the glass by a process called total internal reflection (TIR). By interfering with (i.e. scattering and/or absorbing) the evanescent wave, one may prevent (i.e. “frustrate”) the total internal reflection phenomenon.
Systems employing frustrated TIR to obtain images of biometric prints are generally known in the art. The basic principle of these conventional systems is that light, if incident on an interface going from one medium (with n1) to another medium such as air (with n2<n1), will be totally reflected if the incident angle is large enough. A camera is generally oriented to image the reflected light. The reflected light may form a white background. However, if material (such as a finger ridge) with a relatively high refractive index makes intimate contact with the interface from the air side, then total internal reflection is disturbed and some of the light is transmitted into the contacting finger ridge, instead of being reflected. Thus, this region appears dark in the camera image. The result is a high contrast fingerprint image.
Challenges for Total Internal Reflection Based Fingerprint Interrogation
Conventional TIR based imaging systems suffer a number of drawbacks. For example, conventional TIR systems may capture incomplete fingerprints from dry fingers, because they do not have enough index matching moisture or oil to make intimate contact with the a prism surface (e.g., glass or plastic). The result is that dry finger prints typically appear as strings of intermittent dots, rather than patterns of continuous dark ridge lines.
Another drawback of conventional TIR based imaging systems is that if a finger is too moist, the valleys between the fingerprint ridges are filled with fluid and the low reflectivity contact area overlaps both the ridges and valleys of the finger. The result is a dark “blob” in the image such that few, if any, fingerprint ridges may be discerned from the image.
Yet another drawback of conventional TIR based imaging systems is the “halo” effect. That is, when moisture emanating from the finger condenses nearby on the prism surface. Since the glass or plastic used to make the prism is typically hydrophobic, the condensate typically forms tiny droplets. These droplets partially prevent TIR and thus appear as a shadow or halo around the fingerprints in the image.
Another drawback is that residual oil on the prism surface of conventional TIR based imaging systems may generate unwanted residual fingerprint images. If an operator does not clean the prism surface often, significant residual fingerprint oil patterns may remain on the prism from previous users. False or confusing composite fingerprints may be captured because of the residual oil.
Conventional TIR based imaging systems have prism surface areas that allow ambient light transmission into the system. Optical filters and light shades may be used to help alleviate effects of ambient light, but only to some degree. For example, capturing fingerprints in full sunlight is typically not possible since the ambient sunlight passing into the system dominates the light signal provided by a light source from the device. Ambient light may cause the camera detector to be saturated, and often no fingerprint image can be detected.
Limitations of Conventional Electrical Fingerprint Characterization Methods
Alternative electrical fingerprint characterization approaches that rely on sensing human tissue directly may suffer from low signal-to-noise ratios. Low signals may occur because the un-optimized electrical properties of human tissue may lead to smaller than desired changes in detected resistance, capacitance, etc. as a function of proximity to or pressure against an electrical readout grid. High noise may occur because of spatial and/or temporal variations in human tissue properties independent of the fingerprint profile itself. These high signal-to-noise ratios may effectively blur or obscure the fingerprint image. This may lead to poor spatial resolution of the collected fingerprint image, even if the readout sensor geometry has sufficiently fine spatial resolution.
Many developers of liquid crystal displays have incorporated touch sensitivity to form “touch screens”. These touch screens allow a human finger to control many functions depending on the context. Although low resolution patterns related to an individual's finger are sometimes used to provide basic security and identity functions, it is generally recognized that the present resolution of touch screens is insufficient to produce high quality fingerprints.
Alternative methods that rely on human tissue to directly complete circuits in the readout sensor array depend on highly variable conditions at the tissue sensor interface, such as moisture, sweat, oil, dirt, corrosion, oxidation, variability in the resistance or impedance of human tissue from one person to another, and/or other variable conditions. Repeatedly exposing the electrical contacts to human tissue may reduce the service life of these alternative electrical approaches.
Some electrical fingerprint characterization methods include a film between the finger and the readout grid to protect the electronics. However, these conventional films tend to reduce the sensitivity of the sensing mechanism.